If you're going to continue to run the accounts, as a boxed pair.
I'd go with either IS Boxer ($50/year), or if you're comfortable writing your own scripts Hot Key Net (Free).

If you're boxing, just for the leveling up.
And will be running two separate accounts, once the leveling is done.
Use HKN (if you're comfortable writing your scripts) or Keyclone ($20, lifetime license) if you're not.

IS Boxer has far superior mouse broadcasting.
Gets you instant swapping between master/slave.
And is very user friendly, in that you can just plant a spell on '1' for both toons, and press '1' in-game on either toon which will have them both cast at the active region's target.

If you go with another boxing software.
Use /focus, that's going to be the easiest assist method.
Everyone has a /focus ToonA and everyone has a /focus Toon B.
Toon A Focus, is on the same keybind for everyone; when you push it, they all focus the same toon.
Then you DPS with: /cast [@focustarget] NukeSpellHere
And Heal with: /cast [@focustarget,help][@focustargettarget,help] HealSpellHere
And Follow with: /follow [@focus]



If you're leveling two together...
Via questing or running instances, etc.

Go with double damage, even if you'll play as Heals/Tank at maximum level.
You'll kill faster, and with both regenerating mana... have less down time; not to mention survive harder pulls.




If money is not a consideration...
The cost of an extra Battlechest + WotLK expansion...
Then RAF A > B > C.
Boost toons on B/C, with A.
When you're done RAF, discard C and forget about ever owning that account.
It's still going to be a lot faster than questing with A/B.

You could even do B = Pally, C = Mage.
Then transfer the Mage to B, for really fast leveling... if the goal is maximum toons in minimum time.

If it's: I want to play the game, and enjoy the questing process...
Then RAF is irrelevant, and boosting is detrimental to experiencing the lore and learning the insides of every class and combination.